1. Field of the Invention
The present invention relates to the field of measuring and monitoring patient body fluid levels and more particularly to measuring and long-term monitoring of body fluid accumulation with bioelectrical impedance.
2. Description of the Related Art
Pulmonary edema is a condition that results in intra-thoracic fluid accumulation, particularly accumulating within the lungs. Pulmonary edema results when the normal exchange of oxygen and carbon dioxide is disrupted by increased pressure within the blood vessels of the lungs, forcing fluid into the alveoli. Alveoli that are filled with fluid are thereby prevented from absorbing oxygen, resulting in pulmonary edema.
In most instances, heart problems are the cause of pulmonary edema, a condition often referred to as congestive heart failure (CHF). CHF is a condition in which the heart does not adequately maintain circulation of blood. CHF is characterized by an increase in thoracic fluid, particularly in the lungs wherein pulmonary edema is the result.
CHF may occur when the left ventricle of the heart cannot pump out enough of the blood received from the lungs. As a result, pressure increases inside the left atrium and then in the pulmonary veins and capillaries, causing fluid to be pushed through the capillary walls into the alveoli of the lungs. Various medical conditions and disease states exist that may cause the left ventricle to weaken and eventually fail include: coronary arty disease; cardiomyopathy; heart valve problems; and high blood pressure (hypertension).
CHF may also occur when the right ventricle is unable to overcome increased pressure in the pulmonary artery. This is normally a result from left-side heart failure, chronic lung disease or high blood pressure in the pulmonary artery (pulmonary hypertension). Persistent pulmonary edema may raise pressure in the pulmonary artery and eventually the right ventricle begins to fail. Since the right ventricle has a much thinner wall of muscle than does the left side, the increased pressure backs up into the right atrium and then into various parts of the body, including a buildup of fluid in the pleural space (pleural effusion).
Not all pulmonary edema results from heart disease. Fluid may also leak from the capillaries in the alveoli because the capillaries themselves have become more permeable, even without the buildup of back-pressure from the heart. This condition is noncardiac pulmonary edema. Some conditions or disease states that may cause noncardiac pulmonary edema include: lung infections; exposure to certain toxins such as chlorine, ammonia or nitrogen dioxide; anaphylaxis; smoke inhalation; drug overdose; acute respiratory distress syndrome; and high altitude.
Accurate assessment of this thoracic and/or pulmonary fluid accumulation is critical in assisting with diagnosing the condition and/or disease as well as monitoring the effectiveness of treatment regimens. Electrocardiography (ECG) will reveal a range of information about the heart's function, including inter alia heart rate and rhythm and whether areas of the heart may have diminished blood flow. Echocardiography is another well-known technique to assist in diagnosing heart-related problems that may contribute to pulmonary edema. Transesophageal echocardiography (TEE) may also be used to diagnose heart and central pulmonary artery problems. Cardiac catherization may be used to measure the pressure in lung capillaries.
Generally, a decrease in extracellular fluid within the lungs indicates an improvement in the condition and/or disease while an increase of extracellular fluid within the lungs indicates a worsening of the condition and/or disease.
These conventional methods either require expensive equipment and trained personnel, i.e., cardiac catheterization or echocardiography, or are simply not very accurate in monitoring intra-thoracic fluid accumulation, in particular pulmonary edema. A more accurate and non-invasive technique is highly desirable.
It has long been known in the art that changes in body fluid levels are correlated with overall body changes in impedance. Impedance is a complex quantity, consisting of a resistive or active component and a capacitive or reactive component. Bioelectrical impedance measurement and analysis is made possible by the many complex circuits of the human body, with cells and the interstitial fluid each having distinct electrical characteristics.
Cells comprise membrane-bounded chambers filled with a concentrated solution of nutrients. The cell membrane comprises a non-conductive phospholipid bilayer sandwiched between two layers of conductive protein molecules. The phospholipids are arranged tail to tail around the circumference of the cell membrane, acting as an electrical insulator. The heads of the phospholipids carry a charge, i.e., are polar, while the tails are non-polar. The cell membrane further comprises water-soluble proteins therethrough, creating pores through which water, nutrients, waste, etc., may enter into and exit from the cell. This cell membrane thus functions as a permeable barrier separating the intracellular (cytoplasm) and extracellular (interstitial) components.
The membrane-enclosed cells have electrical characteristics that may form capacitors, and thus have reactance. On the other hand, the extracellular/interstitial fluid environment in which the cells are immersed is primarily resistive in nature.
The first component of impedance, resistive or active impedance (R) is the resistance to the flow of an electric current; a characteristic shared by all substances. Reactance (Z) is the second component of impedance and is the opposition to the flow of electrical current caused by capacitance in biological tissues, particularly cell membranes. Impedance is the vector sum of resistance R and reactance Z, where reactance is the Y coordinate and resistance is the X coordinate. Thus, impedance is equal to the square root of the squared sums of the values of X and Y.
The biological tissue model wherein a cell is immersed in interstitial fluid may be analogized to an electrical circuit having a resistor (interstitial fluid) in parallel with a capacitor (cell bounded by membrane). Reactance Z is inversely proportional to frequency. Thus, reactance Z decreases as frequency increases, and as frequency decreases, reactance Z increases.
The effect of the inverse relationship of reactance and frequency on measuring impedance using the biological tissue model discussed above is that electrical current at very low frequency will not penetrate the cellular membrane, which acts as an insulator in this case. Therefore, very low frequency current passes through the extracellular/interstitial fluid, responsible for the resistive component R of impedance while the reactive or capacitive component Z of impedance will be very nearly zero.
Conversely, very high frequency current causes the capacitive cellular membrane to behave as a nearly perfect capacitor. In this case the impedance reflects a combination of both the resistive component and the reactive component.
Using this information, Subramanyan, et al. and others have shown that both the resistive and reactive components of the body's impedance to flow of a relatively high frequency electrical current may be correlated with the amount of fluid retained by a patient. As the accumulated fluid dissipates with treatment, the resistance R and reactance Z both increase as does the electrical phase angle. See Subramanyan, et al., “Total Body Water in Congestive Heart Failure”, Jour. Asso. Phys. Ind., Vol. 28, September, 1980, pp 257-262; U.S. Pat. No. 5,788,643. These known techniques comprise applying electrodes to two limbs of a patient and then passing a high frequency current between the electrodes. Current, voltage and phase angle are calculated and compared with baseline values to determine whether intervention is required. However, these known techniques measure total body water and do not specifically focus on intra-thoracic or pulmonary fluid accumulation.
Moreover, known impedance measurements in patients are subject to a plurality of artifacts that affect the measurement both during a measurement timepoint (intra) and from timepoint to timepoint (inter). For example, electrode placement from measurement to measurement will differ causing impedance differences, the impedance of the skin changes over time, chest cavity impedance will change substantially, by as much as 300 percent during the respiratory cycle due to the ever-changing volume of air in the lungs, chest cavity impedance also changes by as much as 3 percent during the cardiac cycle due to the constantly changing perfusion levels of the lungs. In addition, simple movement by the patient and/or patient posture changes, both intra-measurement and inter-measurement, introduces motion artifacts that result in changes in the resistive R and reactive Z components of impedance, skewing the results.
Many complicated techniques have been proposed to eliminate the effects of impedance artifacts upon impedance measurements. For example, complex compensation techniques for changing impedance of the skin are discussed in U.S. Pat. No. 5,749,369. Temporal averaging has also been proposed, among other techniques, to eliminate the very large impedance changes due to the changing air volume in the lungs. See, e.g., Eyuboglu, B. M. et al., “In Vivo Imaging of Cardiac Related Impedance Changes,” March 1989, IEEE Engineering in Medicine and Biology Magazine, Vol. 8, pp. 39-45. Moreover, U.S. Pat. No. 5,311,878 suggests numerical techniques to reduce noise in impedance measurements and U.S. Pat. No. 5,746,214 outlines use of different impedances at different electrical frequencies to assist in distinguishing between cardiac and respiratory affects. Each of these known techniques is complex, involving correction of impedance date in at least one aspect.
A more accurate method of accurately assessing impedance, and fluid accumulation, in the human thoracic cavity and lungs would be highly desirable.